rig shifts upon IK application
if you choose the Palm_L_Ik as the target of the Forearm_L, the Forearm_L will stick to the head of the Palm_L_IK, that's why it moves when you switch to Pose mode, if you don't want this to happen, you need, in Edit mode, to put the tail of Forearm_L at the same point than the head of Palm_L_Ik...

By the way, the Deform option is to prevent the mesh to be affected by the bone, it has nothing to do with your problem